FIG. 1 is a cross-sectional view illustrating the fundamental construction of a wet-type multiplate clutch 10. FIG. 1 shows a clutch casing 21, a counterpart hub 22 to which a torque is transmitted, a spline groove 23 formed on the clutch casing 21, a spline groove 24 formed on the hub 22, a piston 25 for pressing separator plates 30 and friction plates 40 against a backing plate 26, a snap ring 27 supporting the backing plate 26, and a sealing ring 28 for the piston 25. The separator plates 30 are maintained in fitting engagement with the spline groove 23, while the friction plates 40 are maintained in fitting engagement with the spline groove 24.
In recent years, there is an ever-increasing demand for improvements in the fuel economy of automobiles. Keeping in step with this trend, there is an outstanding demand for a further reduction in drag toque between friction plates and separator plates during non-engagement of a clutch in an automatic transmission.
Conventional clutches are equipped with friction plates provided with friction linings, each of which has one or more oil grooves having closed end portions to separate the friction plates from their associated separator plates during non-engagement of a clutch and also has one or more oil passages extending radially through the friction lining to feed lube oil onto a friction surface for the prevention of seizure during engagement of the clutch.
To improve the shift response in an attempt to make not only an improvement in fuel economy and but also improvements in engine performance, the clearances between friction plates and their associated separator plates have become smaller recently than before, tending to result in a greater drag torque due to intervening oil films during idling.
With such conventional friction plates, no sufficient drainage of lube oil is feasible on their friction surfaces, thereby failing to satisfactorily meet the demand for a further reduction in drag torque. Especially during rotation in a low-speed range, the drainage of intervening lube oil between the friction plates and the associated separator plates is not sufficient so that the drag torque cannot be reduced.
Lube oil is fed from the radially-inner side of friction plates, and is then drawn onto their friction surfaces. Once the thus-drawn lube oil enters between the friction plates and their associated separator plates, its drainage does not take place quickly. Especially when the clearances between the friction plates and the separator plates are small and the clutch is in a low rpm range, this tendency is pronounced so that during idling, a significant drag torque is produced due to the viscosity of the lube oil between the friction linings and their counterpart separator plates.
When the friction surface of each friction lining is provided with plural second oil grooves opening to a radially-outer side of the friction lining and closed at an opposite end portion thereof (hereinafter called “second oil grooves”), lube oil which has been drawn onto the friction surface from an oilway is smoothly drained to the radially-outer side so that during idling, a drag torque can be reduced. This drag-torque reducing effect is high especially during low-speed rotation. Because the drawn lube oil is smoothly drained, frictional heat which is produced during clutch engagement is also removed smoothly together with the lube oil, so that the heat resistance of the friction lining is improved.
When the friction surface of each friction lining is provided with plural first oil grooves opening to a radially-inner side of the friction lining and having closed radial outer end portions (hereinafter called “first oil grooves”), these oil grooves are effective in keeping uniform the clearances between the friction plates and their associated separator plates during idling owing to the action that separate the friction plates and the separator plates from each other and, when the clutch is disengaged, can smoothly separate the friction plates and the separator plates from each other. These oil grooves are, therefore, also effective in reducing a drag torque during idling.
When a friction lining having these oil grooves is formed into segment pieces and these segment pieces are bonded at angular intervals on one side of a core plate, oil passages are formed between the respective segment pieces such that the oil passages extend through the friction lining from the radially-inner side to the radially-outer side. Accordingly, any extra lube oil is promptly drained through the oil passages, thereby very effectively reducing a drag torque during idling. (See, for example, JP-A-11-141570 and JP-A-2005-76759)
In an initial stage of clutch engagement, however, the lube oil which exists on each friction surface is quickly drained from the friction surface via these oil grooves and oil passages, and therefore, the cushioning effect of the lube oil is reduced to cause abrupt grabbing of the clutch in the initial stage of clutch engagement. The use of this clutch as a clutch, brake or the like in an automatic transmission causes a problem that a shock is produced when the automatic transmission is shifted.